EDIT: And, the short answer beats me to it...

You want to save the mouse position?
For later use in the program, or to the file system?
I'm going to go with the former...

It is very simple to store the mouse variables ( mouseX and mouseY) to a global variable...
You can use a PVector to store both values in one variable, or two separate variables...

Here is an example with PVector:

1. PVector savedMouse; //The saved mouse position
2. 
   
3. void setup() {
4.   size(400, 400);
5.   savedMouse = new PVector(0, 0); //Initialize the PVector
   
6. }
7. 
   
8. void draw() {
9.   println("Current saved mouse position: " + savedMouse.x + ", " + savedMouse.y); //Print the saved mouse position
   
10. }
11. 
    
12. void mousePressed() {
13.   savedMouse.x = mouseX;  //Save the mouse position;
    
14.   savedMouse.y = mouseY;
15. }